1. Field of the Invention
The present invention relates to methods of inspection and apparatus usable, for example, in the manufacture of devices by lithographic techniques. The invention may be applied for example to detect processing faults on semiconductor wafers arising during processing by a lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Two known approaches for inspecting a semiconductor wafer after processing by a lithographic apparatus are: 1. Fast inspection for defects with dense sampling to look for anomalies on the wafer; and 2. Critical Dimension (CD) and Overlay (OV) metrology on a few selected sites where a detailed (and therefore time consuming) measurement is done of the resist profile and the overlay.
For Integrated Metrology (IM) of CD it would be preferable to measure all wafers in a lot and cover as much wafer area as possible since this gives the highest chance of detecting localized process excursions, however the time spent performing such measurements needs to be considered. Move Acquire Measure (MAM) time is a figure of merit for methods of inspecting semiconductor wafers. The MAM time includes: time spent moving the wafer between measurement sites; time spent aligning the measurement target to the inspection apparatus at the measurement site; and time spent acquiring the measurement. MAM time for CD is of the order of 300-3000 ms at best (depending on the application) which is too long to measure all wafers in a lot with sufficient wafer coverage. This increases the chance of missing the relevant locations on the wafer where large process excursions may be present.